The present invention relates generally to rotary machines, and, more particularly, to methods and apparatus to facilitate reducing packing leakage losses in rotary machines.
Steam and gas turbines are used, among other purposes, to generate power for electric generators. A steam turbine has a flow path which typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Compressor and turbine sections include at least one row of circumferentially-spaced rotating blades.
Turbine efficiency depends at least in part on controlling the radial clearance or gap defined between the tips of the rotor blade and the casing, and maintaining a clearance between the rotor and adjacent diaphragm or end packings. If either clearance is too large, steam or gas flow may leak through the clearance gaps, thus decreasing the turbine""s efficiency. Alternatively, if either clearance is too small, the rotor blade tips may undesirably contact the surrounding casing during certain turbine operating conditions. Gas or steam leakage, either out of the flow path or into the flow path, from an area of higher pressure to an area of lower pressure, is generally undesirable.
To facilitate minimizing gas-path leakage, at least some known turbines use a plurality of labyrinth seals. Known labyrinth seals include longitudinally spaced-apart rows of labyrinth seal teeth to seal against high-pressure differentials that may be present in a turbine. However, labyrinth seals may wear-out after extended use, and prior to replacement, worn labyrinth seals may be ineffective for controlling leakage of fluid between the areas of generally high and low pressure. Furthermore, because of changing centrifugal forces induced to the blade tips, and thermal growth differences between the rotating rotor and stationary casing, the effectiveness of the labyrinth seals may be limited as the operating clearances change during periods of acceleration, deceleration, or transient operations during turbine startup. More specifically, during periods of differential centrifugal and thermal growth of the rotor and casing, clearance changes may cause undesirable rubbing of the moving blade tips against the stationary casing. A resulting increase in blade tip clearance may result in additional efficiency loss.
The anchor positions of the stationary parts do not typically align with the rotor thrust bearing and the steady state temperature of the rotor is typically hotter than the shells, therefore the thermal expansions of these components can result in large relative movements between the rotating and stationary parts, especially for the low pressure sections of steam turbines when the trust bearing is placed far from this section. The ability to predict (calculate) these relative axial differential expansions between the rotating and stationary parts, hereinafter referred as xe2x80x9caxial shiftxe2x80x9d, fosters the use of this invention. All design variations of the invention serve to open radial clearances to avoid rubs during start-up (cold condition), when rotor vibrations are highest, and to close (tighten) the radial clearances during steady state (hot condition) to effectively lower secondary flow losses and increase machine efficiency.
In one aspect, a method for sealing between a rotary component and a stationary component. The method including positioning a seal assembly having a plurality of teeth adjacent the stationary component and positioning a packing surface to interface with the rotary component. The packing surface at the rotary component includes at least one step that has an axial width that is approximately equal to a distance the rotary component axially shifts relative to the stationary component during transition from startup to steady state operating conditions.
In another aspect of the invention, a seal assembly for use in a turbine is provided. The seal assembly includes a rotary component including a packing surface and a stationary component including a plurality of teeth extending from the stationary component towards the packing surface. The packing surface including at least one step having an axial width that is approximately equal to a distance the rotary component axially shifts relative to the stationary component when the turbine transitions from a startup to a steady-state condition such that the seal assembly facilitates sealing between the rotary component and the stationary component.
In a further aspect, a turbine is provided. The turbine includes a housing, a rotor shaft rotatably coupled within the housing, a diaphragm coupled to the housing; and a seal assembly extending between the rotor shaft and the diaphragm. The seal assembly includes a rotary component including a packing surface and a stationary component including a plurality of teeth extending from the diaphragm stationary housing towards the packing surface to provide a seal therebetween. The packing surface comprising at least one step having an axial width that is approximately equal to a distance of axial shift of the rotary component relative to the stationary component when the turbine transitions from a startup to a steady state condition such that the seal assembly facilitates sealing between the rotary component and the stationary component.